1. Field of the Invention
The present general inventive concept relates to a heating roller (HR) used to fix a toner image, and more particularly, to a power control method and apparatus to supply an external source power to a heating resistor included in a heating roller to heat the heating roller in an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus, such as a printer or a copy machine, which forms an image of print data on a printing medium by using a developing material such as toner, a toner image corresponding to the print data is fixed onto the printing medium, and the printing medium is then discharged out of the image forming apparatus, thereby obtaining printed matter.
The image forming apparatus may use a heating roller having heating resistors. In this case, in order to perform a fixing operation, a surface temperature of the heating roller has to be maintained around a fixing target temperature, for example, 180° C.
The image forming apparatus is switched to a print mode when the image forming apparatus receives a printing order after power is turned on, or when the image forming apparatus receives the printing order in a standby mode. Here, a time required after the printing order is received and before a first printed matter is discharged is referred to as a first print out time (FPOT).
In order to reduce the FPOT of the image forming apparatus including the heating roller, the surface temperature of the heating roller has to more rapidly reach the fixing target temperature. The heating resistor may be made of tungsten, and may have a variable characteristic in which a resistance thereof is determined in proportion to a heating resistor's temperature equal to or less than a threshold temperature.
FIGS. 1A and 1B are waveform diagrams illustrating a power control principle of a conventional heating roller. Referring to FIGS. 1A and 1B, a voltage (Vin) 110 illustrated is applied to a heating resistor from an external source, causing a current (Ir) 120 to flow through the heating resistor. Further, the current (Ir) 120 is gradually decreased until a heating roller's temperature reaches a threshold temperature. The power control principle of the conventional heating roller has a drawback in that a circuit may be damaged due to an excessive current that may flow through the heating resistor when power is initially or suddenly supplied to the heating roller. In this case, a high current may flow through the heating resistor in the form of an alternating current, thereby exhibiting a deteriorating flicker characteristic. The flicker characteristic is defined as a phenomenon in which power supplied to a peripheral circuit is temporarily weakened.
A threshold resistance of a heating resistor at a threshold temperature (of the heating roller) is intrinsically determined. Here, the lower the threshold resistance, the higher the amount of power that can be supplied through to the heating resistor. Thus, the surface temperature of the heating roller can be rapidly increased. However, when a heating resistor having a lower threshold resistance is used, a higher current flows through the heating resistor when power begins to flow through the heating resistor, thereby causing the aforementioned problems. Eventually, according to the conventional power control principle to rapidly heat a heating roller, a heating resistor has to have a sufficiently low threshold resistance. Thus, due (in part) to the deteriorating flicker characteristic, there has been a limit in reducing a time required to increase a surface temperature of the heating roller up to a fixing target temperature STt.
Furthermore, if the conventional image forming apparatus receives a printing order after the image forming apparatus is turned on, the heating roller can be heated only after a control unit (not illustrated) which controls overall tasks performed in the image forming apparatus, for example, a central processing unit (CPU) of the image forming apparatus, is initialized. Therefore, the aforementioned problem that there is a limit in reducing a warm-up time to print becomes more pronounced when the conventional image forming apparatus receives the printing order before the control unit (not illustrated) is initialized.